TTY: introduce tty_port_destroy

After commit "TTY: move tty buffers to tty_port", the tty buffers are
not freed in some drivers. This is because tty_port_destructor is not
called whenever a tty_port is freed. This was an assumption I counted
with but was unfortunately untrue.

Those using refcounting are safe now, but for those which do not we
introduce a function to be called right before the tty_port is freed
by the drivers.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/include/linux/tty.h b/include/linux/tty.h
index d7ff88f..8db1b56 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -455,6 +455,7 @@
 		const struct attribute_group **attr_grp);
 extern int tty_port_alloc_xmit_buf(struct tty_port *port);
 extern void tty_port_free_xmit_buf(struct tty_port *port);
+extern void tty_port_destroy(struct tty_port *port);
 extern void tty_port_put(struct tty_port *port);
 
 static inline struct tty_port *tty_port_get(struct tty_port *port)